byte to uint8_t for ESP8266 3.0 compatibility

[uutzinger]

1. byte to unit8_t for ESP8266 compatibility
2. created additional functions by splitting measureAirQuality and getBaseline into two functions. This allows the main loop to provide the necessary delay timing. To measure airguality, the main loop cancall startAirquality and then later getAirquality. Original function measureAirquality is still available. Similar approach for getBaseline.

[nseidle]

Thanks for the PR! Please add any new functions to the keywords.txt file so they'll get properly highlighted.